UI: Use empty scene for new scene collection

The user may not want their audio or their display to be captured when
creating a new scene collection.  Make new scene collections default to
fully empty.

obs/window-basic-main.hpp

@@ -114,7 +114,7 @@ private:
 	void          SetupEncoders();
 
 	void          CreateFirstRunSources(obs_scene_t *scene);
-	void          CreateDefaultScene();
+	void          CreateDefaultScene(bool firstStart);
 
 	void          ClearVolumeControls();